This invention arose out of a need to modulate a fluid control pressure from a minimum value to a maximum value in response to an electrical signal of increasing and decreasing amplitude. For several years, proportional control units have been available for use in hydraulic and pneumatic systems. Further, magnetic latching control units (separate from the proportional units) have also been available. None of the available proportional devices or latching devices combine the proportional and latching features into a single configuration. Moreover, the known devices are not configured such that they could perform the combined function even with moderate alterations. The teachings in U.S. Pat. No. 3,740,594, assigned to the same assignee as the present invention, disclose a proportional control unit as well as a separate construction involving a latching control unit. This patent does not disclose the combined proportional and latching features into a single configuration nor does it disclose how the resulting mechanical output might be converted to a hydraulic pressure output.
A review of the known prior art disclosing devices having proportional movement capabilities indicates that for any given value of movement of the armature in the axial direction, forces increase somewhat in proportion to the increase in coil current. It has also been noted that when the current reverts to zero at any position of the stroke, the force generated by the coil current will also return to zero precluding a holding of the armature of the device at a predesignated location due to resistive forces in the system pushing the armature back away from the position whereat the armature was located at the time the current was turned off or due to the magnetic circuitry returning the armature to a preselected position. The prior art discussed above relates only to devices with mechanical outputs, however, general knowledge is available to incorporate these devices into hydraulic circuits for pressure control of either the proportional or latching type.
An investigation of the characteristics of a typical latching device utilizing permanent magnet circuits as well as electrical magnetic circuits revealed that the force to extend the armature increases as the armature moves toward the extended position and the force to retract the armature also increases as the armature moves toward the retracted position. This defines a bistable device that will latch in each of the two limit positions of the armature until coil current is applied to reverse the magnetic field and cause the armature to move in the opposite direction.
It is obviously apparent that none of the above devices exhibit the characteristics required for achieving a proportional performance (i.e. effecting an armature movement that is proportional in its movement characteristics to the current supply to the electromagnetic circuit) and a latching performance (i.e. effecting a latch at one end of the stroke following a proportional travel of the armature). It is also apparent that none of these devices exhibit the characteristics which allow the control of hydraulic pressure in a proportional manner and in the same configuration provide for a latching mode of operation which will hold pressure at a specified level even when coil current is returned to zero. The basic requirements for the overall magnetic circuit, to enable an achievement of the proportional armature travel functions and latching functions, should meet the following criteria:
(a) The proportional control must be initiated at or adjacent to the zero current armature position.
(b) The proportional range of the magnetic circuit must be maximized (desirably it should be 75% of the total stroke).
(c) The total useful stroke of the motor must include a proportional travel that is approaching five times the normal travel of a proportional (nonlatching) control and must also provide an appropriate stroke length to achieve latching.
(d) The magnetic requirement for latching based on magnetic circuit characteristics must be adjacent to the end of the proportional circuit and have a maximum force versus displacement gain approximately one order of magnitude greater than the gain in the proportional range.
(e) The relationship between the permanent magnet circuit and the electromagnetic circuit must be such that delatching will be accomplished within acceptable current limits.
In addition to the magnetic circuit requirements, the hydraulic circuit characteristics should meet the following requirements:
(f) The overall button travel required to provide the needed pressure control will be a function of nozzle and orifice size as well as the control pressure required to operate the second stage. This travel usually is in the range of 0.005 to 0.010 inches.
(g) The force required to control the hydraulic circuit will be a function of nozzle diameter and second stage pressure requirements.
(h) The physical connection between the armature and the poppet valve member must allow the armature to move full stroke (approximately 0.05 inches) while the poppet valve member travel is limited to approximately 0.005 inches.
The analysis becomes even more complex when the armature movement is resisted by a nonlinear force increase as, for example, a poppet valve member moves closer to a valve seat to close off a fluid nozzle. Thus, not only does the magnetic circuit and the electromagnetic circuit need to be varied to compensate for this nonlinear increase in the load on the armature, but also additional considerations need to be reviewed to facilitate a latching of the armature in a position whereat the poppet valve member comes into contact with the nozzle. This particular function is highly desirable in situations where a large earth moving vehicle, for example, has employed therewithin a braking system that must remain fully activated even in view of an electrical power failure on the vehicle.
Accordingly, it is an object of the invention to provide a proportional control device for proportionally controlling the pressure of a fluid within a chamber to enable that pressure to be utilized in a fluid control circuit.
It is a further object of the invention to provide a proportional control device, as aforesaid, wherein general proportionality is achieved between the magnitude of the coil current and the magnitude of the distance between the poppet valve member and the nozzle in a pressure control circuit.
It is a further object of the invention to provide a proportional control device, as aforesaid, wherein following a proportional travel of the armature, additional current supply will cause the armature to move to a latched position.
It is a further object of the invention to provide a proportional pressure control device, as aforesaid, wherein the latching of the armature will cause a control pressure to be maintained at a maximum level even though electrical current may be removed from the coil.
It is a further object of the invention to provide a proportional control device, as aforesaid, wherein the overall size of the permanent magnet and electromagnet circuitry is approximately 2 inches long and 1 inch in diameter.